The following four documents represent the closest technical state to the presently disclosed subject matter.
Document WO2004093461 divulges a video transcoder that receives a coded signal in an initial video signal coding format, decodes this signal so as to generate a decoded signal, extracts a set of estimated motion data on the basis of the initial video signal (still according to the initial format), and generates a second set of estimated motion data based on the first set of estimated motion data. This second set of estimated motion data is coded in a second coding format. It does not undermine the novelty of the disclosed subject matter in question, because unlike the presently disclosed subject matter, where the determination of the best motion vector candidate is based only on the criterion of minimum residue, irrespective of the size of the block, in the aforesaid document the motion vectors are determined using the average of the set of candidate vectors and the selection of the best motion vector candidate is based on the size of the block. A method according to the presently disclosed subject matter can be more effective in selecting the best vector because it results in a residue signal with less energy. Likewise in this presently disclosed subject matter, the determination of the best position with accuracy of half a pixel is carried out through a search in a window which is 1×1 pixels in size, while in the aforesaid document this is obtained by simply shifting to the closest whole position, or half pixel. The search in a 1×1 window, as is the case in the presently disclosed subject matter, allows better motion vectors to be obtained, which result in residual signals with less energy. Furthermore, the aforesaid document does not construct an exhaustive list of all the possible candidates which result from all the reference frames and all the sub-blocks of the macro-block situated in the same spatial position in the H.264/AVC frames, so as to determine the motion vectors (MV) of a macro-block in MPEG-2 format. The use of this list increases the probability of selecting a better motion vector candidate, than in the case of the aforesaid document. The cited document also does not define a way of converting vectors of the “unrestricted motion vectors” type, while the disclosed subject matter in question presents a method based on inversion and temporal mirroring. Likewise the aforesaid document does not present any method to create bi-directional references based on a single motion vector, unlike the present disclosed subject matter, which defines a method based on a combination of the temporal mirroring technique and the application of a scale and refinement factor.
Document US20070030903 divulges a method to transcode an H.264/AVC format to MPEG-2, consisting of or including the following steps: (A) decoding of the H.264/AVC format to generate a frame with macro-block pairs which used a H.264/AVC coding; (B) determining an indicator mode for each pair of macro-blocks; (C) coding the pairs of macro-blocks in a video output in the MPEG-2 format, using either (i) an MPEG-2 field mode coding, or (ii) an MPEG-2 frame mode coding (frame). It has similarities with the presently disclosed subject matter, because both the method divulged in document US20070030903 as well as the method of the presently disclosed subject matter—according to the information provided—consist of or include decoding an H.264/AVC format video input into a set of macro-blocks, using them to produce new coding information, and coding them in the MPEG-2 format. However, it does not undermine the novelty of the present method, which is different because, as described above, it includes determining motion vectors (MV) in an MPEG-2 format, based on other motion vectors in the H.264/AVC format, by creating a list of candidate MVs constituted by all the MVs extracted from the original macro-block in an H.264/AVC format, redimensioning only the candidate MVs that use reference frames that are different from the ones permitted by the MPEG-2. The aforesaid document also does not define a way of converting vectors of the “unrestricted motion vectors” type, while this disclosed subject matter presents a method based on inversion and temporal mirroring. Likewise, the aforesaid document also does not present any method to create bi-directional references based on a single motion vector, unlike this disclosed subject matter, which defines a method based on a combination of the temporal mirroring technique and the application of a scale and refinement factor. Unlike the presently disclosed subject matter, where the best motion vector candidate is determined only on the basis of the criterion of minimum residue, irrespective of the size of the block, in the aforesaid document the motion vectors are determined by using the mean, median or mode of the set of candidate vectors and the selection of the best motion vector candidate can be based on the size of the block. The method of the presently disclosed subject matter is more effective in selecting the best vector because it results in a residual signal with less energy.
Document US20070030905 divulges a video transcoder consisting of or including a first processor, and a second digital video signal processor coupled with the first processor. The second processor has (i) a first module configured to realize an initial operation of decoding an input video sequence in a first format, and (ii) a second module configured to realize a second operation of coding of an output video sequence in a second format, wherein the first and second operations take place in a parallel manner. These operations include reconstructions of each macro-block by means of the motion vectors and motion compensation. It does not undermine the novelty of the presently disclosed subject matter because this disclosed subject matter differs by including, as described above, the determination of motion vectors (MV) in an MPEG-2 format, based on other motion vectors in H.264/AVC format, by creating a list of MV candidates constituted by all the MVs extracted from the original macro-block, in an H.264/AVC format, redimensioning only the MV candidates that use different reference frames from the ones permitted by MPEG-2. The aforesaid document also does not define a way of converting vectors of the “unrestricted motion vectors” type, while this disclosed subject matter presents a method based on inversion and temporal mirroring. Likewise, the aforesaid document also does not present any method to create bi-directional references based on a single motion vector, unlike this disclosed subject matter, which defines a method based on a combination of the temporal mirroring technique and the application of a scale and refinement factor. Unlike the presently disclosed subject matter, where the best motion vector candidate is determined only on the basis of the criterion of minimum residue, irrespective of the size of the block, in the aforesaid document the motion vectors are determined by using the mean, median or mode of the set of candidate vectors and the selection of the best motion vector candidate can be based on the size of the block. A method according to the presently disclosed subject matter is more effective in selecting the best vector because it results in a residual signal with less energy.
Document WO2007124491 divulges a method of removing a motion vector from a group of motion vectors used in a coding process; preparing a list of motion vectors; selecting an initial motion vector from the list of motion vectors; supplying an intermediate motion vector using a process of refining the motion vector, a process that partially uses the initial motion vector; forming a region defined by one or more parameters associated with the initial motion vector and one or more parameters associated with the intermediate motion vector; selecting an additional motion vector from the list of motion vectors; determining whether the additional motion vector points to the region; and modifying a state of the additional motion vector. It does not undermine the novelty of the present disclosed subject matter because the method contained in the said document is only aimed at transcoding motion vectors, while the present disclosed subject matter transcodes both the motion information as well as the interframe coding modes, and because the method of the present disclosed subject matter specifically transcodes between the H.264/AVC and MPEG-2 formats, determining the motion vectors (MV) in an MPEG-2 format, on the basis of other motion vectors in the H.264/AVC format, by creating a list of candidate MVs constituted by all the MVs extracted from the original macro-block, in the H.264/AVC format, redimensioning only the candidate MVs that use different reference frames from the ones permitted by the MPEG-2 format. The aforesaid document also does not define a way of converting vectors of the “unrestricted motion vectors” type, while this disclosed subject matter presents a method based on inversion and temporal mirroring. Likewise, the aforesaid document also does not present any method to create bi-directional references based on a single motion vector, unlike this disclosed subject matter, which defines a method based on a combination of the temporal mirroring technique and the application of a scale and refinement factor.
The presently disclosed subject matter represents inventive activities as compared to the material divulged in the documents WO2004093461, US20070030903, US20070030905 and WO2007124491, because, unlike document WO2007124491, this is a method that can be applied both to motion vectors as well as to interframe coding modes, and also because, unlike documents WO2004093461, US20070030903, US20070030905 and WO2007124491, it includes the determination of motion vectors (MV) in an MPEG-2 format, based on other motion vectors in the H.264/AVC format, by creating a list of candidate MVs comprising all the MVs extracted from the original macro-block in the H.264/AVC format, redimensioning only the MV candidates that use reference frames that are different from those permitted by the MPEG-2 format.
The said documents do not precisely specify the method used to determine motion vectors (MV) in the MPEG-2 format on the basis of other motion vectors in the H.264/AVC format. They only mention some possibilities for converting H.264/AVC format motion vectors to MPEG-2, but do not include the following aspects, which, in their turn, are an integral part of the proposed method.
Main Differences with Regard to the State of the Technique
                A. The creation of a list of candidate motion vectors (MV), constituted by all the MVs extracted from the original macro-block in the H.264/AVC format (1).        B. The procedure described in A only includes the redimensioning of the candidate MVs that use reference frames that are different from those permitted by the MPEG-2 (8) format.        C. Procedure A includes an inverted mirroring method, for MVs that exceed the frame limits (“unrestricted MVs”) in bi-directional frames. It consists of or includes inverting the MVs with regard to both the horizontal and vertical axes, along with exchanging the reference frame for the temporally opposite frame, with regard to the frame to be coded.        D. In bidirectional frames, the MVs with a unidirectional prediction are used to generate bidirectional candidate MVs, combining the MV of the unidirectional prediction with an MV inverted in accordance with the procedure described in C.        E. The selection of the best candidate MV, based on the list created in A, B, C and D, using a criterion of least residue, for example, Sum of Square Differences (SSD=ΣiΣj (f(i,j)−p(i,j))2), or Sum of Absolute Differences(SAD=ΣiΣj | f(i,j)−p(i,j)|), or another criterion.        F. The selected MV is later refined using the same criterion of least residue, in a search window centred on the position described by the selected candidate MV.Exemplary Advantages        
This method can have the advantage of producing better results in terms of image quality, owing to the fact that the selection is based on the criterion of least residue (instead of averages or medians of all the MVs). It presents a more efficient solution than the trivial solution (for example, truncation) in the case of the MVs that exceed the frame edges (“unrestricted MVs”) in B type (bi-predictive) frames. It presents a method to improve the prediction of macro-blocks represented by a unidirectional MV in B type (bi-predictive) frames, enabling its bi-directional prediction.
General Description of the Disclosed Subject Matter
This is an efficient method to transcode (23) H.264/AVC (1) format frames into the MPEG-2 (8) format. The method of transcoding (23) comprises methods of converting both (i) interframe modes of coding (4), as well as (ii) motion information inserted into a coded video, from the H.264/AVC (1) format to the MPEG-2 (8) format. The methods (a) process the coding mode information associated with each temporally predicted H.264/AVC (1) macro-block, and produce new coding mode information, equipped with MPEG-2 (8) semantics; and the methods (b) process the motion information associated with the multiple reference frames, and produce motion information, equipped with MPEG-2 (8) semantics.
The method is also characterized by determining motion vectors (MV) in the MPEG-2 (8) format based on other H.264/AVC (1) format motion vectors, by creating a list of candidate MVs, constituted by all the MVs extracted from the H.264/AVC (1) format macro-block, redimensioning only the candidate MVs that use reference frames that are different from those permitted by the MPEG-2 (8). This procedure also includes an inverted mirroring method for the MVs that exceed the edges of the frame (“unrestricted MVs”) in bi-directional frames, which consists of or includes the inversion of the MVs with regard to both the horizontal and vertical axes, along with exchanging the reference frame for the temporally opposite frame with regard to the frame being coded. The proposed algorithms provide an efficient conversion method, which can be used in personal video recording systems (25), to receive personal video contents and transmit in the MPEG-2 (8) format, through wire connections (27), or wireless connections (26), to a common MPEG-2 decoder.
This method has the advantage of producing better results in terms of the quality of the frame, owing to the fact that the selection is based on the criterion of least residue (instead of on averages or medians of all the MVs). It also presents a more efficient solution than the trivial solution (for example, truncation), in the case of the MVs that exceed the edges of the frame (“unrestricted MVs”) in B type (bi-predictive) frames, and also presents a method to improve the prediction of macro-blocks represented by a unidirectional MV in B type (bi-predictive) frames, enabling its bi-directional prediction.
The current H.264/AVC standard presents a much better compression performance than competing standards, namely MPEG-2. However, the MPEG-2 video standard is still the most common video compression format, and its widespread use in professional and home equipment is expected to last for many years to come, namely in digital television (DTV), personal video recorders (PVR) and digital versatile discs (DVD).
Owing to its higher compression efficiency, the H.264/AVC is becoming increasingly accepted in multimedia applications and services, such as in High Definition Digital Television (HDTV), mobile television (MTV) and the Internet.
The simultaneous use of diverse coding standards undoubtedly raises problems of inter-operationability, because the same type of source material can be available in a format that is incompatible with the target equipment. Moreover: it is very unlikely that the migration of the H.264/AVC technology to MPEG-2, both in terms of professional as well as home equipment will happen in such a short span of time that the problems resulting from the co-existence of both the standards can be ignored. Consequently, transcoding (23) from H.264/AVC (1) to the MPEG-2 (8) format is necessary to maintain retro-compatibility and to facilitate the migration technology.
To date, much of the research and development effort has been focused on constructing efficient transcoding (23) systems from MPEG-2 (8) to H.264/AVC (1), so as to migrate inherited video content to the new format. In contrast, little attention has been paid to the problem of retro-compatibility.
A trivial implementation of a transcoder (23) is a cascade of an H.264/AVC decoder and an MPEG-2 coder. However, such a method completely ignores the H.264/AVC coding information integrated into the flux of bits (1), which is the result of optimization decisions between the rate of bit transfer and distortion, with a view to coding each block with the highest possible efficiency. Using such a trivial transcoder (23), the decoded H.264/AVC (1) frames have to be fully coded in MPEG-2 (8), as though no previous coding information existed. This enormously increases the complexity of the transcoder (23) and the necessary computational resources, without achieving a better coding efficiency.